A chain of peptide containing linear sequences of amino acid linked by peptide bonds best represent the
- (a)Primary structure
- (b)Secondary structure
- (c)Tertiary structure
- (d)Quaternary structure
Correct — A, Primary structure. The primary structure of a protein is exactly this: the linear sequence, or order, of amino acids in the polypeptide chain, joined one to the next by peptide bonds. It is the most basic level of protein organisation and it determines all the higher levels of folding. A chain of amino acids in a set order linked by peptide bonds is, by definition, the primary structure.
- (b)Secondary structure — Secondary structure is the local coiling or folding of that chain into regular patterns such as the alpha-helix or beta-pleated sheet, held by hydrogen bonds — it is a level above the plain linear sequence.
- (c)Tertiary structure — Tertiary structure is the overall three-dimensional shape of a single folded polypeptide, stabilised by interactions between side chains (disulfide bridges, ionic and hydrophobic bonds) — not the simple linear sequence.
- (d)Quaternary structure — Quaternary structure is the assembly of two or more separate polypeptide chains (subunits) into one functional protein, as in haemoglobin's four chains — well beyond a single linear sequence.
Proteins are organised at four levels. Primary structure is the linear order of amino acids joined by peptide bonds. Secondary structure is local folding into alpha-helices and beta-sheets held by hydrogen bonds. Tertiary structure is the full 3-D fold of one chain. Quaternary structure is the joining of several chains into one protein.
Match the description to the level. The phrase 'linear sequence of amino acids linked by peptide bonds' names the most basic arrangement, with no mention of folding, 3-D shape or multiple chains — that is the primary structure. Each higher level adds something (hydrogen-bonded coils, 3-D folding, multiple subunits), so anything beyond a plain sequence is ruled out.
- Primary structure = the linear sequence of amino acids joined by peptide bonds.
- Secondary structure = alpha-helix and beta-pleated sheet, stabilised by hydrogen bonds.
- Tertiary structure = the overall 3-D shape of a single polypeptide chain.
- Quaternary structure = two or more polypeptide subunits assembled (e.g. haemoglobin).
A plain linear sequence linked by peptide bonds is the primary structure — answer (a).
- Reading 'chain' or 'peptide bonds' and jumping to a higher level — the primary structure is precisely that chain.
- Confusing secondary (local coils/sheets) with tertiary (whole 3-D shape).
Protein-structure questions give a description and ask for the level — anchor each level to one defining feature.
No directly related past PYQ was found.
- practice — not a real PYQ
The alpha-helix and beta-pleated sheet of a protein represent its
- (a)Primary structure
- (b)Secondary structure
- (c)Tertiary structure
- (d)Quaternary structure
Answer(b) Secondary structure — regular local folding held by hydrogen bonds.
- practice — not a real PYQ
Amino acids in a protein are joined together by
- (a)Glycosidic bonds
- (b)Peptide bonds
- (c)Hydrogen bonds only
- (d)Ester bonds
Answer(b) Peptide bonds — the covalent links between successive amino acids.